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Re: rules on vector sequence

  • To: mathgroup at smc.vnet.net
  • Subject: [mg98083] Re: [mg98018] rules on vector sequence
  • From: Filippo Miatto <sottosupremo at gmail.com>
  • Date: Sun, 29 Mar 2009 02:48:58 -0500 (EST)
  • References: <200903281041.FAA04380@smc.vnet.net> <op.urix2tgitgfoz2@bobbys-imac.local>

wow i have to study how this works! ; )
thanks a lot!

On Mar 28, 2009, at 11:57 PM, DrMajorBob wrote:

> Here's a solution:
>
> Clear[sums, insertions]
> insertions[x : {__List}] := Union @@ insertions /@ x
> insertions[x_List] := Union[Insert[x, 0, #] & /@ Range[1 + Length@x]]
> sums[1] = {};
> sums[2] = {{-2, 2}, {-1, 1}, {1, -1}, {2, -2}};
> sums[4] = Module[{nonZero = {-2, -1, 1, 2}},
>   Union @@
>    Permutations /@
>     Join[Select[Tuples@{nonZero, nonZero, nonZero, nonZero},
>       0 == Total@# &], insertions@insertions@sums@2]
>   ];
> sums[len_?Positive] := sums[len] = insertions@sums[len - 1]
>
> Timing[Length@sums@17]
>
> {7.9271, 86224}
>
> sums@3
>
> {{-2, 0, 2}, {-2, 2, 0}, {-1, 0, 1}, {-1, 1, 0}, {0, -2, 2}, {0, -1,
>  1}, {0, 1, -1}, {0, 2, -2}, {1, -1, 0}, {1, 0, -1}, {2, -2, 0}, {2,
>  0, -2}}
>
> Length@sums@4
>
> 60
>
> Length@sums@5
>
> 220
>
> s6 = sums@6;
> Length@s6
> Total /@ s6 // Union
> Total /@ Unitize@s6 // Union
> Length@s6 == Length@Union@s6
>
> 600
>
> {0}
>
> {2, 4}
>
> True
>
> Bobby
>
>
> On Sat, 28 Mar 2009 05:41:26 -0500, Filippo Miatto <sottosupremo at gmail.com 
> > wrote:
>
>> Dear all,
>> I need to generate all the vectors, of a given length N, that have
>> these three properties:
>>
>> 1- the entries have to be among -2,-1,0,1,2
>> 2- the sum of all the entries has to be 0
>> 3- only two or four of the entries can be different from 0
>>
>> do you have any suggestions on how i can do that? i tried something
>> but without success.. expecially i don't know how to implement the
>> third rule..
>> thank you in advance!
>> Filippo
>>
>
>
>
> -- 
> DrMajorBob at bigfoot.com



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